Nanotubes promise fuel from water

PORTLAND, Ore.  Sandia National Laboratories recently demonstrated that hollow organic nanotubes, married to an inorganic catalyst, can harness sunlight to turn water into pure hydrogen and oxygen. By 2006, Sandia researchers hope to have prototypes from which a new kind of solar cell could be made that would convert water into fuel.

Such cells might replace fossil fuels in automobiles and thus reduce the United States' dependence on foreign oil.

"If we get lucky and it works [efficiently], then we will try to engineer cells out of these things," said Sandia researcher John Shelnutt. "Then there is going to be a lot of work for EEs."

Organic nanotubes are used throughout nature to transport electrons and to convert light into energy. In humans, for example, porphyrin nanotubes provide the power by which hemoglobin forms new proteins. The Sandia researchers believe they can harness the same mechanism to power automobiles with water.

"Porphyrins serve in any function where energy has to be harvested," said Shelnutt. "The porphyrins themselves are well-understood, and there is even a chlorosomal rod [in nature that is closely related to porphyrin nanotubes]. It's a nanorod that acts as a light-harvesting agent in photosynthetic bacteria. So our device mimics that."

Shelnutt's group showed that porphyrin nanotubes can be prepared in aqueous solution by virtue of the ionic self-assembly of two oppositely charged porphyrins. By adding metal ions to the solution, the researchers were able to coat the outside of a nanotube with catalytic platinum as well as grow a gold wire  complete with a contact ball for connecting the hydrogen-harvesting part of the nanodevice to the part that harvests oxygen  down the middle of the two connected nanotubes.

"That serves as the connector between the two pieces of the nanodevice," said Shelnutt. "The tube itself is photocatalytic and generates hydrogen. All we really have to do to finish the device is attach the gold ball [contact] out on the end of it to something like tungsta, a small bandgap inorganic photocatalyst material that's known to evolve oxygen."